Highest FPS?

The GTX 750 Ti + i5-4690K at stock frequency can surpass 1000 FPS easily on the lowest settings with Optifine.

As far as a theoretical maximum framerate is concerned, this is dependent on how LWJGL 2 handles framerates exceeding the signed 32-bit integer limitation of 2,147,483,647.

Even if you're operating with a 60 Hz monitor, you can still benefit from > 60 FPS in-game since your system will still draw the frames at the calculated framerate despite the fact that your monitor will only be able to display a new frame every 1/60th of a second.

And yes, the human eye can differentiate between 30, 60, and 120+ FPS when there's something moving in view. You're not seeing the world in measurable frames and your eye isn't rendering the world in the same way as a display uses interlacing/progressive scanning. Your vision is based on how much light you can take in and how fast your brain can gather and interpret the information gathered by your eyes. There's no direct conversion between the human eye and a digital display of any sort.

I'll make a new thread about this with some diagrams to better explain what I'm rambling about.